There are a number of applications in which conductive materials must be selectively applied to the surface of an insulating substrate. One example is printed circuit boards that have conductive lines plated thereon. These conductive lines may be fabricated by bonding a metallic strip to the substrate; however, it is preferred that the lines be plated directly on the insulating material.
There are a number of known techniques for the electroless plating of metal on insulating substrates. The processes are referred to as electroless because they do not require that an electrical current be passed through the object being plated.
Among these prior art techniques are methods utilizing ultraviolet light to selectively pattern metal onto an insulating substrate. Typically, the spatial distribution of the plated metal on the surface of the substrate is determined by exposing the surface to ultraviolet light after coating the surface with a catalyst, referred to as a photo-promoter. The plating methods are often grouped into "positive" and "negative" methods. Positive methods are those in which metal will be plated in those regions exposed to the ultraviolet light. Negative methods are those methods in which metal will be deposited in those regions which are not exposed to the ultraviolet light.
De Angeleo, et al. (U.S. Pat. No. 3,562,005) describe a method for selectively patterning metal onto an insulating substrate. In the method described by De Angeleo, et al., a photo-promoter such as stannous chloride is used as a photosensitive catalyst. In this procedure, a stannous chloride solution is applied to the surface of the material to be plated. The portions of the surface which are not to be plated are then exposed to ultraviolet light having a wavelength less than 300 nm. Following the exposure to the ultraviolet light, the surface is exposed to a salt of a precious metal such as palladium chloride. In those areas which were not exposed to the ultraviolet light, the following reaction takes place: EQU SnC1.sub.2 +PdC1.sub.2 .fwdarw.SnC1.sub.4 +Pd(1)
The areas which were exposed to the ultraviolet light show no palladium formation, since the absorbed stannous chloride is photochemically oxidized to stannic chloride. Stannic ions cannot reduce the palladium ions in the palladium chloride to metallic palladium. Once the metallic palladium layer is formed, other metals may be electrically plated onto the locations covered by the palladium using conventional plating techniques. De Angeleo, et al. describe processes based on the precious metals palladium, platinum, gold, silver, osmium, indium, iridium, rhenium and rhodium.
Although these processes work effectively for many purposes, they do have some deficiencies. For example, it is very important in these processes that the catalyst be carefully patterned onto the substrate surface. Sometimes the catalyst is inadvertently applied in an area that should not contain metal. In addition, the conductive lines produced utilizing these processes oftentimes do not have the strength is necessary for use in a printed circuit board.
One alternative process uses a laser beam for forming the conductive circuit pattern. In this process, a metal layer is applied covering the entire substrate surface. This layer is then patterned by selectively removing the metallization in predefined areas. The metal is removed using a laser.
The metal layer may be applied to the substrate using any of a number of processes. For example, the substrate may be coated with SnC1.sub.2 and then immersed in a PdC1.sub.2 bath. This results in a layer of metallic Pd being deposited on the surface of the substrate. After the precious metal layer is applied to the substrate, the layer is patterned by utilizing a laser beam to remove the metal layer. Thereafter, conductive metal lines are plated on the patterned metal layer in accordance with known techniques.
Typically, CO.sub.2 or YAG lasers are utilized to thermally vaporize the metal layer in the areas of the substrate that are to be devoid of metal. These laser processes, although effective for many purposes, are oftentimes not as precise or reliable as needed when patterning conductive lines for a printed circuit board. The above-mentioned types of laser processes rely on heat for cutting through the material to be processed. Hence, the processes depend on relatively long wavelength light which can be difficult to control in providing minimum depth cuts in a material.
More particularly, in providing conductive lines on the substrate surface, as before mentioned, the plated metal (such as palladium) on the substrate is very thin, on the order of 10 to 40 angstroms. Thus, it is very important that the laser penetrate the metal layer a very short distance to avoid degradation of the substrate. The above-mentioned processes are difficult to control to these depths. In addition, these processes leave a large percentage of their energy in the substrate due to thermal activity. In so doing, the area surrounding the processed portion of the substrate is damaged by the thermal effects. Hence, the above-mentioned laser processes are not as effective as desired when a small area is to be patterned.
Excimer lasers are finding increasing use as an alternative to the above-mentioned laser processes for the selective patterning of a metal layer that is on the surface of an insulating substrate. Excimer lasers are those lasers producing relatively short wavelength light, such as in the ultraviolet range. Excimer lasers typically operate at a wavelength of 180 to 370 nm at 50 to 200 millijoules of power. Of particular importance to this process is that excimer lasers remove material by ablation rather than by melting or vaporization.
In many cases, this ablative mechanism results in a higher degree of precision than can be achieved through other types of laser processes. What is meant by the ablative mechanism in this application is that when an ultraviolet light of sufficient energy irradiates a surface, the energy interacts with the material of the surface and results in the decomposition and ejection of the surface layer.
Although a detailed understanding of the complex ablative mechanisms has yet to be developed, all recent theories agree on the following two points. (1) The ablative material is removed layer by layer on a pulse-by-pulse basis of the excimer laser; and (2) the majority of the energy in the laser pulse is used in bond breaking and ejecting ablative material from the substrate. Consequently, very little energy remains in the substrate, and the thermal diffusion to surrounding areas is greatly reduced.
The physical mechanism of ablation differs remarkably from the thermal mechanisms of other laser processes for material removal. Hence, excimer lasers offer new capabilities in the area of electroless plating.
The ablative mechanism gives rise to several characteristics which are unique to excimer laser operation. First, the materials can be removed with extremely high precision and excellent edge definition. Second, there is significantly less charring or burning of the surrounding material. Third, there is a minimum heat effective zone, that is, there would be little if any distortion of the substrate using the laser. Finally, the definition of patterns by mask imaging can be done rather than by translation of a focused spot of laser light along a trajectory on the substrate surface, as used in non-ablative laser processing. Thus, an entire area on the substrate can be processed at once.
Another key feature of the excimer laser processing technique is that the absorption of the energy occurs within a localized region near the surface of the irradiated material. Therefore, there is little opportunity for thermal diffusion to the surrounding material. Consequently, the exposed material can be removed while the underlying material is left virtually untouched, even in the case of very thin layers of metal.
Although in principle this can also be achieved using CO.sub.2 or other type lasers, in practice, the thermal nature of conventional laser processing will ultimately do considerable destruction to the remaining substrate. Hence, the use of excimer lasers for patterning a substrate which has a thin metal layer plated thereon provides clear advantages over other types of laser processing techniques.
However, a particular problem with all of the above laser processing techniques is that the overall adherence of the resultant conductive layer to the substrate is oftentimes unacceptably low. Hence, the quality and integrity of the circuit pattern is also compromised.
More particularly, it has been found that when a circuit pattern is provided utilizing known ablative laser processing techniques, oftentimes the circuit pattern can be easily peeled away from the surface of the substrate. Hence, the printed substrates have to be carefully handled to ensure the integrity of the circuit pattern. In addition, when electrical connections are made to the circuit pattern, there is the danger that the connections are defective or faulty due to the adherence problem.
Since there may be thousands of these circuit patterns utilized in conjunction with certain applications, it is important to provide a reliable and effective method and apparatus for providing a conductive circuit pattern that has a higher adherence to the substrate than previous systems obtained when utilizing the ablative laser process.
Broadly, it is an object of the present invention to provide an improved system and method for conductive circuit patterning utilizing laser ablation.
It is also an object of the present invention to provide a system and method that produces strengthened conductive lines compared to those lines produced by previously known laser techniques.
These and other objects of the present invention will become apparent to those skilled in art from the accompanying detailed description.